In automobile coating, as well as in other fields which require coatings with good decorative quality with simultaneous good corrosion protection, it is normal to coat the substrates with a number of coatings. At the present time, finishing coatings are applied in two steps. First, a pigmented coat (base or first coat) is applied, followed by a transparent finish, normally a clear coat, containing no, or only transparent pigments. To impart a metallic effect to the finish, the base coat contains metallic pigments.
Presently in industrial automobile coating, coating compositions are used which contain a high level of organic solvents. For economical and ecological reasons substantial efforts have been made to reduce the organic solvents to the greatest extent possible. Thus, coating compositions have been developed containing mainly water as the dispersing medium and only small quantities of organic solvents. Specifically, U.S. Pat. No. 3,862,071 discloses a water-dilutable binder system for metallic enamels containing a water-soluble acrylic copolymer and hexamethoxymethylmelamine, partially neutralized with amines and thus dispersed. The essential property of a paint binder for metallic pigments, i.e., the fixation of the metallic pigment particles in a position favorable for optical effect, is achieved in this case by the addition of a water-insoluble copolymer. According to GB-PS No. 2,073,609, the desired optical effect is achieved by using internally crosslinked copolymer dispersions ("microgels") in the base coat. According to EP-A No. 1 00 29 597, the desired optical effect is achieved by providing a partially dispersed character to the water-soluble acrylic copolymer by using a balanced ratio between hydroxy and carboxy groups. Efforts have also been made to obtain the required performance of the base coat by co-employing polyurethane dispersions as disclosed in DE No. 32 10 051 or by the introduction of cellulose esters as disclosed in DE No. 32 16 549.
In all of those previously known systems the rheological properties necessary for the orientation and fixation of the highly anisotropic metallic pigment flakes to give the desired metallic effect are obtained through the presence of partially dispersed binder and additional components which influence the rheology, such as thixotropic agents or extenders. These measures, however, either adversely influence the storage stability and the applicational properties, particularly when application is with electrostatic high rotation atomizers, or the weather resistance of the film.